It is known to break up (fracture) and stimulate formations surrounding underground wells to increase the rate of recovery of hydrocarbons from the wells. According to the prior art, fracturing and stimulation, by means of wireline technology as well as fluid-carrying strings, have been time-consuming and expensive. By the use of a wireline, bridge plugs with a diameter that is slightly smaller than the inner diameter of the well path have been pumped down the well to below a perforated zone which is to be stimulated and/or fractured. The entire annulus outside the wireline above the plug must be pressured up. The bridge plug is generally dependent on a clean well path in order to be moved in the well, and then in particular to be pumped past perforated casings. Therefore, there has generally been a need to clean the well after each perforating operation. As the wireline has a limited breaking strength, the well plugs must usually be left in the well and be drilled out later, which reduces the effective diameter of the well after the operation. It is not possible to fracture and/or stimulate an isolated zone by means of wireline technology. Fluid-carrying strings like production tubing or coiled tubing have been used for stimulation and fracturing as well. A challenge has been to provide enough power for a fracturing and/or stimulating tool downhole. The power loss in long downhole electric transmission cables may be considerable, and the upper permissible transmission voltage has been set by official requirements. Both downhole generators and electromotors must be limited in size because of the limited diameter of the wellbore. Downhole electromotors are therefore limited in power. It is a challenge to provide sufficient forces for carrying out different operations downhole.